Deletion of a Pathogenic Mutation-Containing Exon of COL7A1 Allows Clonal Gene Editing Correction of RDEB Patient Epidermal Stem Cells

Recessive dystrophic epidermolysis bullosa is a severe skin fragility disease caused by loss of functional type VII collagen at the dermal-epidermal junction. A frameshift mutation in exon 80 of COL7A1 gene, c.6527insC, is highly prevalent in the Spanish patient population. We have implemented geneediting strategies for COL7A1 frame restoration by NHEJ-induced indels in epidermal stem cells from patients carrying this mutation. TALEN nucleases designed to cut within the COL7A1 exon 80 sequence were delivered to primary patient keratinocyte cultures by non-integrating viral vectors. After genotyping a large collection of vector-transduced patient keratinocyte clones with high proliferative potential, we identified a significant percentage of clones with COL7A1 reading frame recovery and Collagen VII protein expression. Skin equivalents generated with cells from a clone lacking exon 80 entirely were able to regenerate phenotypically normal human skin upon their grafting onto immunodeficient mice. These patientderived human skin grafts showed Collagen VII deposition at the basement membrane zone, formation of anchoring fibrils, and structural integrity when analyzed 12 weeks after grafting. Our data provide a proof-of-principle for recessive dystrophic epidermolysis bullosa treatment through ex vivo gene editing based on removal of pathogenic mutationcontaining, functionally expendable COL7A1 exons in patient epidermal stem cells.The study was mainly supported by DEBRA International—funded by DEBRA Austria (grant termed as Larcher 1). Additional funds come from Spanish grants SAF2013-43475-R and SAF2017-86810-R from the Ministry of Economy and Competitiveness and PI14/00931 and PI17/01747 from the Instituto de Salud Carlos III, all of them co-funded with European Regional Development Funds (ERDF)

Clinically Relevant Correction of Recessive Dystrophic Epidermolysis Bullosa by Dual sgRNA CRISPR/Cas9-Mediated Gene Editing

Gene editing constitutes a novel approach for precisely correcting disease-causing gene mutations. Frameshift mutations inCOL7A1 causing recessive dystrophic epidermolysis bullosaare amenable to open reading frame restoration by non-homologous end joining repair-based approaches. Efficient targeteddeletion of faulty COL7A1 exons in polyclonal patient keratinocytes would enable the translation of this therapeutic strategy to the clinic. In this study, using a dual single-guide RNA(sgRNA)-guided Cas9 nuclease delivered as a ribonucleoprotein complex through electroporation, we have achieved veryefficient targeted deletion of COL7A1 exon 80 in recessivedystrophic epidermolysis bullosa (RDEB) patient keratinocytescarrying a highly prevalent frameshift mutation. This ex vivonon-viral approach rendered a large proportion of correctedcells producing a functional collagen VII variant. The effectivetargeting of the epidermal stem cell population enabled longterm regeneration of a properly adhesive skin upon graftingonto immunodeficient mice. A safety assessment by next-generation sequencing (NGS) analysis of potential off-target sitesdid not reveal any unintended nuclease activity. Our strategycould potentially be extended to a large number of COL7A1mutation-bearing exons within the long collagenous domainof this gene, opening the way to precision medicine for RDEB.The study was mainly supported by DEBRA International, funded by DEBRA Austria (grant termed as Larcher 1). Additional funds came from Spanish grants SAF2017-86810-R (to M.D.R.) and PI17/01747 (to F.L.) from the Ministry of Economy and Competitiveness and Instituto de Salud Carlos III, respectively, both co-funded with European Regional Development Funds (ERDF) ERA-NET E-RARE JTC 2017 (MutaEB) and CIBERER (grant termed as Murillas- TERAPIAS ER2017)